I'm assuming you're viewing this in the Preview. The Preview has some issues with Stores at the moment. This is an open issue we are working on. If you export your project and view in your own browser from your local web server...and given your "url" is setup properly...you will see that your configuration is absolutely correct and loads perfectly. The only other option you may want to change (depending on the behavior you want in the end) is to make the mode "local" instead of "remote" (the default).
I am having the same problem and I am using a browser. I can see in firebug that my data store is loading. One thing that confuses me is that in the ui file in the combo definition, the store name has quotes around it. Any suggestions on why the combo is empty?